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Antibody Introduction

What are Antibodies?


         Antibodies are produced by B lymphocytes (B cells) when an unknown substance enters the body. These cells have antigen receptors on their surface which help to distinguish foreign substances (antigens). The basic structure of each antibody molecule is basically similar to the antigen receptors of the B cell.


        These proteins are made up of two pairs of polypeptide chains (amino acids linked together by peptide bonds) that include two heavy chains and two light chains. These four polypeptide chains give antibodies an elastic Y shape. The stem of the Y comprises of one end of each of two similar heavy chains, while the remaining part of the heavy chain and the light chain are present on the arm of Y. The two light chains of antibodies are also similar. Both heavy chains and light chains are composed of the constant region and variable region. The stem and the bottom of the arms of the Y-shaped molecule are justly identical in all classes of antibodies and thus these regions are called the constant region. While the tips of the arms are composed of variable amino acid sequences and they are called the variable region. These tips participate in the antigen binding site of the antibody. Hence each antibody has two alike antigen-binding sites present at the end of each arm and these sites differ significantly among antibodies. The antigen-binding properties of antibodies are mainly concentrated in short parts within the variable domains (VH and VL) which display a higher amount of variability. These are known as complementarity-determining regions (CDRs) or hypervariable regions because of their direct association with antigen binding (Zouali,2001).


        The whole structure of the antibody is divided into two parts: fragment antigen-binding (Fab fragment) and fragment crystallizable region (Fc region). Fab portion is an antigen binding region of the antibody and it is made of one variable and one constant domain of both chains. These domains shape the antigen binding site at the amino terminus of the monomer. Fc portion is the tail part of the antibody and it interacts with Fc receptors and a few proteins of the complement system. This property of Fc portion enables the antibody to trigger the immune system (Paul 3rd,1993).

Fig(a): Structure of antibody molecule.

How antibodies work:

        When a foreign substance (antigen) enters the body, it will stimulate the immune response. The antigen will bind to the B lymphocytes (B-cell) surface and thus activates the B cell to rapidly divide and mature into a set of similar cells known as a clone. Many millions of antibodies are produced in the bloodstream and lymphatic system by mature B cells that are called plasma cells.


        The antibodies attack and defuse antigens that are similar to the ones that activated the immune reaction. Antigens are attacked by antibodies when these protein molecules bind to the antigens. For example, antitoxins are the antibodies that bind to a toxin and neutralize it by varying its chemical composition. Some antibodies can render invading microbes steady and prevent them from entering the body cells by attaching to them. While in some cases, the antibody-coated antigen is subjected to a chemical chain reaction with complement proteins. The complement response can either stimulate the lysis of the attacking microbe or can entice microbe-killing hunter cells that phagocytose the microbe. Once started, antibody production lasts for numerous days until all antigen molecules are attacked and removed. Antibodies provide long-lasting immunity against the specific antigen by remaining in circulation for several months (Steward,2012).

Types of Antibodies:

Antibodies are classified into five types according to their constant region as it determines the isotype. Those five types are designated as  IgGIgMIgAIgD, and IgE. The types of antibodies vary not only in their constant region but also in function.

  • IgG antibody:

IgG is the most common antibody and it is present majorly in the tissue fluids and blood. It is composed of two identical γ (gamma) heavy chains of approximately 50 kDa and two identical light chains of almost 25 kDa. The heavy chains and light chains are made up of four domains (VH, CH1, CH2, and CH3) and two domains (VL and CL), correspondingly. IgG isotypes are subdivided into four subclasses (IgG1-4) depending on their function and structure. Among these subtypes, IgG1 is the most abundant subtype in serum.

IgG antibodies provide protection against bacterial and viral infections. This antibody provides long-term protection as it can remain for months and years after the infection (Bengten et al.,2000).

  • IgM antibody:

IgM antibody is the first line of defense against all infections and its molecular weight is about 900k Da. It is secreted by plasma cells as pentamer. It is constructed of five of six units that include two light chains and two heavy chains. This antibody has ten antigen binding sites and it is mainly involved in agglutination reactions.

  • IgD antibody:

The molecular weight of this antibody is about 180k Da. It constitutes 0.2% of the total immunoglobulins in the body. This antibody was discovered when patients developed multiple myeloma. It is majorly involved in the induction of antibody production ((Bengten et al.,2000).

  • IgA antibody:

This antibody is found in external secretions like breast milk, saliva, intestinal fluid, tears and milk. This is dimeric in shape. This antibody has four antigen binding sites and it has two subclasses that are IgA1 and IgA2.It provides protection against inhaled and ingested pathogens

  • IgE antibody:

The molecular weight of this antibody is 200kDa and it is involved in hypersensitivity reactions. It binds to basophils and mast cells and thus participates in the immune response. Monomers of IgE comprise two heavy chains (ε chain) and two light chains, with the heavy chain having 4 Ig-like constant domains (Cε1-Cε4) (Platts-Mills ,2001).

Basic formats of antibody:

There are three general formats of antibodies:

  • Chimeric antibody

Such antibodies are directly created by genetic engineering, in which the immunoglobulin (Ig) variable domains of a particular mouse hybridoma are joined to human Ig constant domains. They can be used for further humanization (Kontermann and Dübel ,2010).

  • Bispecific antibody

This antibody can supersede the specificity of an effector cell for its regular target and readdress it to destroy a target that it would else disregard. Diverse cytotoxic cells express various triggering receptors. Therefore, by changing the specificities of the effector and target binding regions, a diversity of effector reactions can be fixed against many kinds of target cells. On the other hand, the complete variety of effector functions (i.e. complement activation, ADCC, and phagocytosis) can be deliberated by pointing one binding specificity to serum immunoglobulin (Kontermann and Dübel ,2010).

  • Recombinant antibody

These are monoclonal antibodies formed by recombinant DNA technology. Recombinant antibodies are extensively used in biomedical science and medicine and the biomedical field due to their high sensitivity and reproducibility.


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